Optical shield of transparant intermeshed grooved panels useful on overhead projects

ABSTRACT

A glare control shield having particular utility in an overhead projector, consists of fully transparent matching grooved plates which are disposed between stage and Fresnel condensing lens of the projector and with the grooves generally directed toward the position of the operator. Other applications are described.

BEL/Z76. 01*23-73 OR United States Patent 1 1 Appledorn 51 Jan. 23,1973

( 1 OPTICAL SHIELD OF TRANSPARANT I INTERMESHED GROOVED PANELS USEFUL ONOVERHEAD PROJECTS [75] Inventor: Roger H. Appledorn, White BearLake,Minn.

[73] Assignee: Minnesota Mining and Mannfacturing Company, St. Paul,Minn.

[22] Filed: Aug. 10, 1970 [21] Appl. No.: 62,356

Related U.S. Application Data {63] Continuation-impart of Ser. No.712,262, March 1-1,

1968, abandoned.

[52] U.S. Cl ..350/276 R, 350/167, 353/38, 353/98, 353/102 [51] Int. Cl...G02b 27/00 [58] Field of Search, ..350/276 R, 167; 353/38, 98, 353/102[56] References Cited UNITED STATES PATENTS 3,511,563 5/1970 Erwin..350/276 R lJX 3,126,786 3/1964 Appledorn ..353/98 3,255,665 6/1966Weiher et al ..350/276 R X 3,393,034 7/1968 lmai ..350/276 R X 3,438,6994/1969 Secger .350/276 R 2,976,759 3/1961 Bleuer ..350/276 R PrimaryExaminerDavid Schonberger Assistant Examiner-Toby H. KusmerAttorney-Kinney, Alexander, Sell, Steldt & Delahunt [57] ABSTRACT Aglare control shield having particular utility in an overhead projector,consists of fully transparent matching grooved plates which are disposedbetween stage and Fresnel condensing lens of the projector and with thegrooves generally directed toward the position of the operator. Otherapplications are described.

6 Claims, 4 Drawing Figures PATENTEUJAH23 I975 3.712.713

- SHEET 1 [IF 2 F/Q'i. Z

PATENTEDJAN 23 1975 SHEET 2 OF 2 I I N VEN TOR. Rowe/inflame OPTICALSHIELD OF TRANSPARANT INTERMESIIED GROOVEI) PANELS USEFUL N OVERHEADPROJECTS This application is a continuation-in-part of copendingapplication Ser. No. 712,262 filed Mar. ll, 1968 and now abandoned.

This invention relates to optical shielding devices useful in theelimination of extraneous rays of light. In one important aspect itrelates to an improvement in overhead projectors and to the means foraccomplishing the same. There is provided a fully transparent opticalshield member composed of two intermeshing, identically ridged andgrooved, fully transparent panels.

A form of overhead projector which has enjoyed widespread acceptance forclassroom use, for example as described in U.S. Pat. No. 3,126,786,comprises, in order, a source of intense illumination, a condensing lenswhich is preferably in the form of a Fresnel lens, a flat stage, and afocusing projection head. Means for minimizing heating effects may beincluded if desired. In use, a projection transparency is laid on thestage and the image is focused on a screen. The operator stands at theside or a rear corner of the projector and facing the audience, andwhile speaking may point to significant portions of the transparency, ormark the same as desired, the pointer and marks being visible on thescreen to the watching audience. During this procedure, the transparencyis fully accessible to the operator. However the intensity of extraneouslight from the projector is so great as to interfere severely with theability of the operator to view both the transparency and the audience.Either he is temporarily blinded by the glare so that he cannot see theaudience, or he may employ dark glasses to protect his eyes and thenmust remove the glasses to view the audience, or he finds it necessaryto reduce the intensity of light from the source and thereby makes theprojected image less easily visible to the audience.

The effect of extraneous light, obtained by internal reflection at thecondensing lens, is generally disturbing to the operator, and isparticularly distressing in the case of projectors employing a flatFresnel condensing lens. The structure of such lenses inherently resultsin a concentration of light visible along that diameter of the lenswhich extends toward, or is in line with, the operator, and which hasbeen aptly described as the propeller effect" because of thepropeller-like shape of the visible area of high light intensity and themovement of the beam as the viewer circles the projector.

In U.S. Pat. No. 3,511,563 there is described and claimed an overheadprojector having a glare-reducing shield wherein shielding isaccomplished by the incorporation of thin opaque louvers which aresubstantially parallel to the projection light rays. The louvers aredisposed along the glare path, i.e. across that diameter of the Fresnellens which extends toward the operator, and are so arranged as toprevent or greatly reduce glare at the operators position. Thetransparency remains fully visible to the operator, and the intensity ofthe projected image remains at a high level. Thus the operator isenabled to glance momentarily at the transparency for purposes ofmarking or pointing to the same, and then immediately return his glanceto the audience, without any temporary blindness or any visualdiscomfort, while the projected image remains clearly visible to theaudience.

Another form of light control panel, togethe'r with other applicationstherefor, is described in U.S. Pat. No. 3,255,665. The panel is in theform of two intermeshing ridged and grooved transparent panels. Asomewhat similar structure is described in U.S. Pat. No. 3,393,034. Inboth cases one of the internal surfaces is rendered opaque, so that aconsiderable fraction of a beam of incident light normal to the surfaceof the panel is eliminated from the emitted beam.

Another similar form of light control panel, shown in U.S. Pat. No.3,438,699, employs two intermeshing identically ridged and groovedpanels each of which is fully transparent. However the structure byinternal reflection prevents the passage of light rays incident withinsome few degrees of normal. Since in a typical overhead projector almostthe entire projected beam falls within the indicated angles, the panelwould obviously be worthless as a glare control shield for use therein.

It has now been found possible substantially to eliminate harmful glarefrom an overhead projector without any significant loss in lightintensity at the viewing screen, by employing as a glare control shielda pair of fully transparent intermeshing identically ridged and groovedpanels as will now be more fully described in connection with theappended illustrative drawing, in which:

FIG. 1 is a view, in perspective and partly cut away to show detail, ofa popular commercial form of overhead projector modified in accordancewith the principles of the present invention,

FIG. 2 is a partial vertical cross-section of the glare control shieldemployed therein,

FIG. 3 is a similar view of a combined glare control shield and Fresnelcondensing lens, and

FIG. 4 is a diagrammatic representation of the path of selected lightrays seen from the front of the projector.

The projector 10 of FIG. 1 comprises a case 11 containing a lamp andreflector assembly accessible through a door 12 in the front wall of thecase and connected through a switch 13 with a cord and plug assembly 14for connection to a source of electric power. The case also contains fanmeans for cooling purposes. A bevel-edged stage 15 supported in a frame23 forms a top for the case and a support for projection transparencies.A projection head 16 is supported above the center of the stage on asupport 17 and is vertically adjustable for focusing of the projectedimage by means of knob 18 and associated gearing.

Beneath the stage 15 there is mounted a compound Fresnel condensing lens19. A glare-control shield 22 is disposed within the frame 23 betweenthe stage and the lens. The several components are assembled and held inplace within the frame in known manner, i.e. by means of appropriateledges, spacers, and retaining lugs.

Placing the shield very close to the position of the transparency ofwhich a projection is desired is fully effective in controlling glarebut has the undesired effect of focusing on the screen theglare-controlling pattern. It is therefore preferable to place theshield somewhat below the surface of the stage. A preferred structureincludes the glare control structure with the Fresnel condensing lensstructure in a single assembly, thereby reducing the number of opticalsurfaces required and correspondingly increasing the proportion of thelight reaching the projection screen.

The glare-control shield 22 is shown on a larger scale in FIG. 2 toconsist of mating or intermeshing identically ridged and grooved panels24 and 25 separated by a narrow space 26. For best results the ridgesand grooves are parallel to a line extending from the center of thestage to the position occupied by the operator, and which ordinarily isnear the front comer R as indicated in FIG. 1 and at a distance justsufficient to permit easy hand contact with the stage while keeping theshoulder out of the projected beam. Adequate control of glare mayordinarily be obtained with the ridges and grooves extendinglongitudinally from side to side of the stage area; and this orientationis usually preferable since it is equally effective for bothright-handed and left-handed operators. Variable orientation of theshield may be achieved by simply removing the same and replacing it in adifferent orientation; or the shield may be given a circular outline,rotatably supported in a suitable circular frame within the frame 23,and oriented by partial rotation using a protruding handle member, toplace the ridges and grooves in the most appropriate position for anyparticular position of the operator.

Good results have been achieved with a glare control shield as describedin connection with FIG. 2 mounted midway between the stage and the lens19 and with the longitudinal dimension of the ridges and groovesparallel with the front edge of the frame, i.e. with each ridge andgroove extending from side to side of the shield. The ridges and groovesare formed at an angle, a, of 110 and at a recurring spacing, s, of 20mils (0.02 inch). Each panel desirably has a thickness of about 60 milsto insure adequate structural strength and rigidity. The two may beformed into a unitary article by sealing around the edges, forconvenience in handling and to prevent soiling of the angular surfaces.

The shield is conveniently made from flat sheets of transparentthermoplastic materials, e.g. methyl methacrylate polymer or celluloseacetate butyrate, by pressing with a polished metal mold at elevatedtemperature. The two ridged surfaces are merely laid together, thenormal slight irregularities in the transparent surfaces beingsufficient to maintain a degree of separation adequate for opticalpurposes.

The angle of the prismatic ridges is such that light rays entering frombelow and not too far from perpendicular to the plane of the peaks ofthe ridges pass through the shield, whereas glare rays, or rays ofextraneous light approaching at a considerable angle to said plane andgenerally along said grooves are totally reflected at the prism-airinterface and are prevented from emerging as glare. The particular angleemployed in forming the ridges, while being generally of the order of110, may be different for materials of different refractive index or fordifferent specific purposes. Angles of from about 100 to about 120 havebeen shown to be effective. The spacing S is limited only bymanufacturing considerations, the closest possible spacing beingordinarily preferred. The outer surfaces of the shield are parallel andsmooth.

Improved efficiency ofillumination is achieved in the structure shown inFIG. 3 by the elimination of two glass-to-air interfaces and the slightloss of transmission normally occasioned thereby. In this structure thelower panel of the glare control shield and the upper panel of thecompound Fresnel condensing lens are combined in a single element 27,the upper shield panel 24 and the lower lens element 20 remaining thesame as in the structure of FIGS. 1 and 2. Similarly, the upper panel 24of the shield may serve as the stage 15, although with some loss ofclarity in the projected image as previously noted.

P10. 4 illustrates the effect of the shield 22, constructed as describedin connection with FIG. 2, in permitting passage of all useful rayswhile eliminating harmful glare-producing rays directed to either sideof the projector. The bundle of primary rays indicated by the limitingand normal rays 41, 42, 43 from source 44, are refracted in the Fresnellens 19 and directed toward the projection head 16. In a typicalprojector as here described, the outer rays 41, 42 are at an angle ofabout l5-l7 to the normal, as indicated in the drawing. Secondary rays45, 46, 47, 48 originate mainly by reflection at the lens 19. Rays suchas indicated by full lines 45, 46, and which are at an angle with theaxial ray 43 of not more than about 40 degrees, are not affected. Rayswhich would normally be emitted at angles of less than about 40 andgenerally along the direction of the ridges and grooves, such asindicated by dotted lines 47, 48, are stopped by internal reflection atthe shield. These include the harmful glare rays.

Where the angle a between adjacent faces is made less than about 1 10,the limiting angle to the normal at which the extraneous rays areprevented from passing the shield is increased, until at interfacialangles less than about much or all of the primary beam is prevented fromreaching the projection head. With greater interfacial angles thelimiting angle conversely is increased, and at interfacial anglesgreater than about the glare control area is so small as to be of nopractical value in the operation of the overhead projector.

For convenience in manufacture it is usually desirable that all of theintermeshing ridges and grooves of the two panels are constructed at thesame uniform interfacial angle. Thus in the shield of FIG. 2, and asused in conjunction with FIG. 4, each ridge face forms an angle of 35with the outer flat face of the panel. In some instances it may be founddesirable to slant the ridges and grooves to one side; and suchstructure is also contemplated as coming within the ambit of theinvention. As an example, an incident axial ray after refraction at theentering surface, may be at an angle of 10 to the normal; the two facesof each groove will then be at angles of 25 and 45 respectively with thesurface of the panel, so that the axial ray will again bisect theincluded angle.

Glare shields as here described and illustrated, while 'havingparticular utility in connection with overhead projectors, are alsohighly useful in other light control areas, e.g. windows, vehicleheadlights, beacons etc. Again, the useful light rays incident to theshield at angles near the nonnal are fully transmitted whereas theundesirable glare rays, incident at considerably greater angles to thenormal and generally along the axis of the grooves and ridges, areinternally reflected and prevented from being transmitted through theshield. As a specific example, a windowpane formed as described inconnection with the shield of FIG. 2 and having an interfacial angle ofabout 100, with the grooves and ridges disposed vertically, provides aclear view at most viewing angles but effectively prevents entry ofdirect sunlight with the sun facing the window at an angle greater thanabout 50 above the horizon.

What is claimed is as follows:

1. An optical shield having parallel smooth outer surfaces andconsisting essentially of two fully transparent intermeshing identicallylongitudinally ridged and grooved panels wherein the adjacent faces ofthe ridges or grooves are disposed at an included angle of at leastabout 100 and not greater than about 120 wherein said faces are alldisposed at substantially the same angle with respect to the smoothouter surfaces.

2. The shield of claim 1 wherein said faces are disposed at an includedangle of about 1 l0.

3. The shield of claim 2 wherein said ridges and grooves are at auniform constant spacing of not greater than about 2 mils.

4. The shield of claim 2 wherein said panels are constructed of methylmethacrylate polymer.

5. An overhead projector including a light source, a stage forsupporting a transparency containing projection images, a projectionhead, and a condensing lens for directing light from said source throughsaid stage to said projection head for projection of said images onto aviewing screen, and characterized by the inclusion, in a positionbetween said lens and said stage, of an optical shield consistingessentially of two fully transparent intermeshing longitudinally ridgedand grooved panels wherein the adjacent faces of the ridges or groovesare disposed at an included angle of about 1 10 and with the ridges andgrooves directed toward the sides of said projector.

6. The overhead projector of claim 5 wherein said condensing lens isincluded as a component of said shield.

1. An optical shield having parallel smooth outer surfaces andconsisting essentially of two fully transparent intermeshing identicallylongitudinally ridged and grooved panels wherein the adjacent faces ofthe ridges or grooves are disposed at an included angle of at leastabout 100* and not greater than about 120* wherein said faces are alldisposed at substantially the same angle with respect to the smoothouter surfaces.
 2. The shield of claim 1 wherein said faces are disposedat an included angle of about 110*.
 3. The shield of claim 2 whereinsaid ridges and grooves are at a uniform constant spacing of not greaterthan about 2 mils.
 4. The shield of claim 2 wherein said panels areconstructed of methyl methacrylate polymer.
 5. An overhead projEctorincluding a light source, a stage for supporting a transparencycontaining projection images, a projection head, and a condensing lensfor directing light from said source through said stage to saidprojection head for projection of said images onto a viewing screen, andcharacterized by the inclusion, in a position between said lens and saidstage, of an optical shield consisting essentially of two fullytransparent intermeshing longitudinally ridged and grooved panelswherein the adjacent faces of the ridges or grooves are disposed at anincluded angle of about 110* and with the ridges and grooves directedtoward the sides of said projector.
 6. The overhead projector of claim 5wherein said condensing lens is included as a component of said shield.